In the semiconductor industry, the Si/Si1-xGex heteroepitaxial materials system is of strong interest for future microelectronic applications because the electronic properties of lattice mismatched heterostructures can be tailored for a variety of applications by exploiting band offsets at the interfaces. The most popular application of the Si/Si1-xGex system is heterojunction bipolar transistors (HBTs) that require deposition of a pseudomorphic, i.e., compressively strained so that the in-plane lattice parameter of the layer matches that of the Si substrate, compositionally graded Si1-xGex layer onto the Si substrate. Metal oxide semiconductor field effect transistors (MOSFETs) and modulation-doped field effect transistors (MODFETs) require Si layers under tensile strain to obtain proper conduction band offsets at the interface that enable the formation of a 2D electron gas in the Si quantum well which results in extremely high-electron mobility (on the order of about five to ten times larger than in unstrained Si at room temperature). Si layers under tensile strain are obtained by epitaxial growth on a strain-relaxed Si1-xGex buffer layer (x=0.150.35). As mentioned in P. M. Mooney, Mater. Sci. Eng. R17, 105(1996) and F. Schaeffler, Semiconductor Sci. Tech. 12, 1515 (1997), the strain-relaxed Si1-xGex buffer layer in conjunction with the Si or SOI substrate constitute the so-called “virtual substrate”. It is noted that the term “SiGe” is used sometimes herein to refer to the Si1-xGex layer.
The growth of the strain-relaxed Si1-xGex buffer layer itself is a challenging task since strain relaxation involves controlled nucleation, propagation and interaction of misfit dislocations that terminate with threading arms that extend to the wafer surface and are replicated in any subsequently grown epitaxial layers. These defects are known to have deleterious effects on the properties of electronic and optoelectronic devices. The crystalline quality of the relaxed SiGe layer can be improved by growing compositionally graded buffer layers with thicknesses of up to several micrometers. By using such a technique, the threading dislocation (TD) density in an epitaxial layer grown on top of a buffer layer was reduced from 1010 to 1011 cm−2 for a single uniform composition layer to 106 to 5×107 cm−2 for a graded composition buffer layer. The major drawback of thick SiGe buffer layers (usually a 1-3 micrometer thickness is necessary to obtain greater than 95% strain relaxation, when x=0.3) is the high threading dislocation density and the inhomogeneous distribution of threading dislocations over the whole wafer surface. Some regions have relatively low threading dislocation densities and primarily individual threading dislocations; but other areas contain bundles of threading dislocations as a result of dislocation multiplication which creates dislocation pileups (see, for example, F. K. Legoues, et al., J. Appl. Phys. 71, 4230 (1992) and E. A. Fitzgerald, et al., J. Vac. Sci. and Techn., B10 1807 (1992)). Moreover, blocking or dipole formation may occur, in some instances, due to dislocation interactions (see E. A. Stach, Phys. Rev. Lett. 84, 947 (2000)).
Surface pits that tend to line up in rows are typically found in the latter areas, thus making these regions of the wafer unusable for many electronic devices. Electronic devices on thick graded Si1-xGex buffer layers also exhibit self-heating effects since SiGe alloys typically have a much lower thermal conductivity than Si. Therefore, devices fabricated on thick SiGe buffer layers are unsuitable for some applications. In addition, the thick graded Si1-xGex buffer layers derived from dislocation pileups have a surface roughness of 10 nm on average, which typically makes such buffer layers unsuitable for device fabrication. For example, it is impossible to use these layers directly for wafer bonding. For that purpose an additional chemical-mechanical polishing (CMP) step is required.
Various strategies have been developed to further reduce the threading dislocation density as well as the surface roughness including:
1) The use of an initial low-temperature (LT) buffer layer grown at 450° C. and subsequent layer growth at temperatures between 750° and 850° C. This prior art method makes use of the agglomeration of point defects in the LT buffer layers that occurs at the higher growth temperatures. The agglomerates serve as internal interfaces where dislocations can nucleate and terminate. As a result, the misfit dislocation density that is responsible for the relaxation is maintained, while the threading dislocation density is reduced. LT buffer layers can only be grown by molecular beam epitaxy (MBE); this prior art approach cannot be implemented using UHV-CVD.
2) The use of substrate patterning, e.g., etched trenches, to create small mesas, approximately 10-30 micrometers on a side. The trenches serve as sources/sinks for dislocations to nucleate/terminate. When a dislocation terminates at a trench, no threading dislocation is formed; however, the misfit segment present at the Si/SiGe interface contributes to strain relaxation. The major drawback with this prior art method is loss of flexibility in device positioning and the loss of usable area. Moreover, it is difficult to obtain high degrees of relaxation (greater than 80%).
Neither the conventional graded buffer layer methods to achieve strain-relaxed Si1-xGex buffer layers for virtual substrates, nor the alternative approaches to reduce the density of threading dislocations described above provide a solution that fully satisfies the material demands for device applications, i.e., a sufficiently low-threading dislocation density, control over the distribution of the threading dislocations and an acceptable surface smoothness.
In some cases, He ion implantation has been employed in forming relaxed SiGe layers. Ion implantation of He into semiconductors is well-known to form bubbles that can be degassed and enlarged (Ostwald ripening) during subsequent annealing (see, for example, H. Trinkaus, et al., Appl. Phys. Lett. 76, 3552 (2000), and D. M. Follstaedt, et al., Appl. Phys. Lett. 69, 2059 (1996)). The bubbles have been evaluated for uses such as gettering metallic impurities or altering electronic properties of semiconductors. Moreover, the bubbles have also been evaluated as sources for heterogeneous dislocation nucleation.
It has also been shown that the binding energy between bubbles and dislocations is quite large (about 600 eV for a 10 nm radius of the bubble) and that the interaction of He bubbles with dislocations significantly alters the misfit dislocation pattern. It consists of very short (<50 nm) misfit dislocation segments rather than the longer (greater than 1 μm) ones that occur in graded buffer layer growth. The interaction of He bubbles with dislocations also significantly changes the relaxation behavior of strained Si1-xGex layers. Moreover, the degree of relaxation is greater compared to an unimplanted control sample when the same heat treatment is applied to both samples. To achieve significant strain relaxation, a dose of 2×1016 cm−2 He implanted about 80 nm below the Si/SiGe interface is required (M. Luysberg, D. Kirch, H. Trinkaus, B. Hollaender, S. Lenk, S. Mantl, H. J. Herzog, T. Hackbarth, P. F. Fichtner, Microscopy on Semiconducting Materials, IOP publishing, Oxford 2001). Although the strain relaxation mechanism is very different from that which occurs in graded buffer layers, the threading dislocation density remains unsatisfactorily large (>107 cm2 at best for Si0.80Ge0.20). Lower threading dislocation densities are obtained only when little strain relaxation occurs.
In view of the drawbacks mentioned-above with prior art approaches for fabricating strain-relaxed Si1-xGex buffer layers on Si substrates as well as on silicon-on-insulator substrates (SOI), there exists a need to develop a new and improved process which is capable of fabricating strain-relaxed Si1-xGex buffer layers on Si or silicon-on-insulator (SOI) substrates having a reduced threading dislocation density, a homogeneous distribution of misfit dislocations and a remarkably low surface smoothness.